Dark Days: The Casting When Bruce Wayne regenerated his body and mind using the Final Invention, he received a vision of multiple impossible versions of himself dying over and over again. As he investigates the mystery, it becomes apparent that Dionesium, the metal which had made The Joker so indestructible, has an other-dimensional origin. In the Batcave, The Joker reveals to Duke Thomas and Hal Jordan that Duke was born with Dionesium in his system, and that was why Bruce chose him. He also claims that the machine Batman is building will end the world. In the end, Bruce Wayne's actions set the stage for a hidden world 3,000 miles below Gotham City to emerge to the surface. The Dark Days are over. The Dark Knights are coming.